The present invention relates to a method and a device for machining elongate, non-rotationally symmetrical workpieces in the form of turbine blades using a steady rest that supports the workpiece in accordance with the preambles of the first and eighth patent claims.
When elongate workpieces are mechanically machined, they are often clamped on the two end faces. Since such a clamping procedure allows the workpiece to rotate about the longitudinal axis by means of suitable axes of rotation, the workpiece can be machined on all of the longitudinal sides without having to rechuck. For specific workpiece geometries, for example, the milling of turbine blades, such a chucking operation is necessary, because the workpiece has to be machined continuously over the entire periphery of the longitudinal sides. In the case of other workpieces the workpiece can be finish-machined in only one chucking operation (in contrast to the clamping on a longitudinal side). As a result, time is saved and accuracy is gained, because errors arising between multiple chucking operations are avoided.
With such long and relatively fragile components, such as turbine blades, there is often the problem that such workpieces exhibit a very low rigidity that is much less than the rigidity of the machine that is used. On the one hand such components yield, due to the force of gravity and the machining forces, a feature that results in geometric deviations of the workpieces; and, on the other hand, a high rigidity is necessary for a stable cut in the mechanical machining operation. It is known to support the rotationally symmetrical workpieces with a steady rest for the turning operation. This arrangement dramatically reduces the flexibility of the workpieces.
For example, U.S. Pat. No. 4,177,701 A and DE 10 2007 029 492 B3 describe such steady rests for the turning operation of rotationally symmetrical workpieces. These steady rests have two jaws that can be pivoted in relation to each other and that are configured for receiving the rotationally symmetrical workpiece. The steady rests have roller bearings on their inside diameters; and the work pieces are mounted and supported in a rotational manner by these roller bearings. These steady rests do not lend themselves to machining elongate workpieces with three dimensional free form surfaces, as is the case, for example, with turbine blades.
DE 360222A describes a machine tool for machining elongate turbine blades. In this case the workpiece is clamped on one end; and the central part of the workpiece is held by a rotary part of a steady rest. The steady rest and the rotating spindles are arranged in such a way that they can be rotated vertically. At the same time the steady rest cannot be directed in the direction of the workpiece and is secured on the vertical axis. The steady rest can be moved vertically by means of a screw spindle. An automatic control unit makes it possible to lock and unlock the workpiece in the steady rest. The rotary part of the steady rest is provided with sliding fingers in order to hold the workpiece; and these sliding fingers can be moved in a resilient manner against the workpiece and can be blocked by means of a device. In this case five follow rests are provided for the simultaneous machining of 5 workpieces; and these five follow rests can be moved vertically between a bottom fixed support bracket and an upper support bracket on a counter column by means of a slide for the purpose of clamping the workpiece on the end side. The five turbine blades are inserted transversely through the steady rests, which were unlocked beforehand and moved to the one end of their runway; and these five turbine blades are clamped between the two support brackets. The milling heads that are inserted into the rotating spindles begin the machining at the upper end of the blades; and at the same time the slide with the steady rest is also brought to the suitable height, so that the steady rests can hold the workpieces in the immediate vicinity of the working range of the tool; and the workpieces are clamped into the steady rests by means of four fingers. The machining begins, while the milling heads, mounted on the rotating spindles, engage with the workpieces at their upper end and experience a descending feed motion. The rotary parts of the steady rests are entrained in rotation by means of the blades. Hence, there is no separate rotary drive of the steady rests. For a defined period of time the steady rests stay on the same plane; and the slide, which carries them, is blocked on this plane. In order to prevent the milling heads from moving too far from the region of the steady rests, after they have descended, the steady rests descend by a predefined step. To this end, the machining operation and the rotating motion of the blades stop; the steady rests are unlocked; the slide carrying the steady rests is released; the slide descends by one step; there is an additional blocking; the steady rests are clamped anew on the workpieces; and the machining operation and the rotating motion of the blades resume. The result of this arrangement is that a continuous and, hence, effective machining of the workpieces in not possible. Furthermore, the entire setup for supporting the workpiece(s) is distinguished by a complicated design configuration.
A clamping device for the peripheral-sided clamping of workpieces having an arbitrary circumferential contour is proposed in DE 195 39 488 A1. In this case there are two mutually opposite profiled clamping jaws, which are disposed on a base body and exhibit clamping surfaces. Each of these clamping surfaces is formed by a plurality of rams, which can be fluidly moved independently of each other against the workpiece to be clamped and can be locked in the clamping position. The base body is mounted in a holding device by means of bearing rollers in such a way that the base body can be rotated about a vertical axis that is directed towards the clamping device. The base body is driven via a gear rim, which is disposed on the periphery of the base body, and via corresponding drive wheels by means of a drive motor. Both the base body and the holding device have outwards extending recesses in order to insert the workpiece, thus increasing the risk that high forces will cause the workpiece to spring back. The workpiece is clamped by means of two slides that are arranged opposite each other and can be moved in relation to each other. These slides are provided with profiled clamping jaws in the direction of the workpiece. Each profiled clamping jaw has a plurality of hydraulically adjustable rams, which can be adjusted with respect to the workpiece in order to clamp the workpiece. In order to make, for example, a turbine blade, the first step is to finish-machine a holding end in a machining center on a corresponding blank. Then the blank is chucked at this holding end; and the curved blade surfaces are produced in one or more machining operations in the customary way. Since the opposite holding end that still has to be machined is located relatively far away from the chucked holding end, its machining would present a problem without additional measures. Therefore, the holding device is then moved in the longitudinal direction until it is positioned next to the machined blade surfaces to be machined. To this end there are longitudinal guides. At this point the holding device is moved transversely to the longitudinal direction of the partially machined turbine blade on the transverse guides, as a result of which this partially machined turbine blade moves through the recess as far as up to the chucking opening. This transverse movement necessitates a larger amount of working space. At this point the holding device is positioned in such a way that the longitudinal axis of the partially machined turbine blade coincides with the axis of rotation of the base body. At this point the turbine blade is clamped in the base body with the aid of the profiled clamping jaws. Now the still uncut holding end of the turbine blade can be machined. To this end the partially machined turbine blade can be rotated in the required way by means of the drive motor. In this case this rotation can also be accomplished by means of the chucked, already machined holding end of the turbine blade. After the machining operation, the profiled clamping jaws are released, and the holding device is moved laterally away from the turbine blade. At this point the next turbine blade can be chucked; and the described procedure can be repeated. Since the machining operation has to be interrupted in order to support by means of the steady rests, this feature also constitutes a drawback for the profitability of the machining operation.
Moreover, numerous solutions for clamping workpieces in the form of turbine blades are known from the prior art (DE 10 2004 056 142 A1, DE 10 2005 001 555 A1, DE 100 26 829 C2). These prior art solutions comprise a plurality of clamping elements, which can be clamped against the workpiece surface, but do not allow them a rotational motion of the workpiece.
The publication DE 28 46 851 A1 also discloses a machine tool for machining turbine blades, wherein the workpiece is supported by a rotary part of a steady rest. The aim of this solution is to automate the displacement processes, the unlocking and the locking of the steady rest on its support and on the workpiece. In this case the steady rest is always supposed to be in the working range of the tool, in order to rule out as far as possible the generation of vibrations. In this case the workpiece can rotate in a fixed part of the steady rest. At the same time the machine has devices for automatically controlling the locking and unlocking of the workpiece in the rotary part of the steady rest. And the displacement of the steady rest is controlled. Thus, the steady rest is not actively driven in a rotatable manner, but rather the rotary parts of the steady rests are entrained in rotation by the blades. As a result, torsions in the workpiece may occur; and these torsions in turn may lead to undesired variations in the tolerances.
Furthermore, the steady rest does not move over the clamping point in order to insert the workpiece.
DE 100 26 829 A1 describes a device for securely clamping a workpiece having uneven surfaces. In this case a clamping element or an element, which is coupled in a drive relationship with the clamping element, is provided with a thread, thereby assigning to the linear feed movement of the clamping element a rotating motion of the respective threaded element in relation to the clamp block; with a rotation actuator, which is coupled to the rotatable threaded element for its rotary drive; and with engagable and disengageable means, which can be assigned to at least one clamping element in order to stop the rotatable threaded element by jamming and/or friction, said means comprising a preferably elastic friction element, in particular a jamming element, which allows a torque-free stopping of the rotatable threaded element. In this case, too, there is no rotary drive of the clamping element.
A steady rest for machining rotary parts or crankshafts is described in the publication DE 10 2009 009 056 A1. The workpiece, which is rotationally symmetrical at the clamping point, is supported by the steady rest in order to compensate for the workpiece deflection; and this steady rest can be moved along the machining axis in order to change the workpiece over a clamping point. The steady rest is moved along the machining axis by means of a control unit. During the machining of a crankshaft, the steady rest rotates passively along. The entire process of positioning the steady rest, of clamping and centering the workpiece runs automatically and is controlled by a control unit. It is indicated, as a matter of fact, that the steady rest can be used with a suitable drive as a center drive, but only for rotationally symmetrical workpieces or more specifically crankshafts; and there is no pointer to a specific embodiment. In any event this solution does not lend itself to machining non-rotationally symmetrical workpieces in the form of turbine blades, which have a non-rotationally symmetrical cross section at the clamping point of the steady rest.
The object of the present invention is to develop a method and a device for machining elongate non-rotationally symmetrical workpieces in the form of turbine blades, wherein the workpiece is supported by means of a steady rest, and wherein a simple design configuration of the device as well as an improved machining cycle are guaranteed.
This engineering object is achieved by means of the features disclosed in the first and eighth patent claims.
Advantageous embodiments will be apparent from the dependent claims.
The method for machining elongate, non-rotationally symmetrical workpieces in the form of turbine blades is carried out with the use of a first clamping point for a first end of the workpiece and a second clamping point for a second end of the workpiece, wherein the workpiece is supported by means of a steady rest, which has clamping elements for clamping the workpiece on its non-rotationally symmetrical cross section, wherein, according to the invention,
after clamping the workpiece between the first and the second clamping point, the steady rest moves with its open clamping elements along the longitudinal axis of the workpiece into a supporting position; and
during the movement of the steady rest into the supporting position and/or in the course of changing the supporting position of the steady rest, a collision between the open clamping elements of the steady rest and the workpiece is prevented by means of a program-controlled rotation of a rotary part of the steady rest.
For loading and unloading the workpiece, the steady rest moves at least partially over the first or second clamping point in such a way that the region between the first and the second clamping point is accessible for inserting and clamping the workpiece. As a result, it is possible to move the steady rest only longitudinally to the workpiece and to dispense with a time-consuming and complicated transverse guide that is cost intensive and requires a larger amount of installation space. Furthermore, the solution according to the invention simplifies the process flow.
After clamping the workpiece between the first and the second clamping point, the steady rest moves along the longitudinal axis of the workpiece into a supporting position. At the same time during the movement of the steady rest, collisions with the workpiece are prevented in an advantageous way by means of a program-controlled rotation of the steady rest.
When the steady rest is in the supporting position, said steady rest clamps the workpiece by means of one or more clamping elements, whereupon the machining of the workpiece begins.
Preferably the clamping elements of the steady rest are designed in the form of hydraulic clamping elements. In order to actuate said clamping elements, the steady rest is rotated into a docking position, in which a connection to the corresponding hydraulic elements can be made and can be separated. In this docking position the clamping elements are loaded with a hydraulic pressure by means of the hydraulic elements; and, as a result, the clamping elements are actuated and transferred into a clamping position, so that the workpiece is chucked and clamped in the steady rest. At the same time a collision between the open clamping elements of the steady rest and the workpiece is prevented by means of the program-controlled rotation of the steady rest.
Then the connection to the hydraulic elements is released and the clamping is still retained; and then the machining of the workpiece begins.
In order to machine the region of the workpiece that the steady rest covers, the clamping of the clamping elements of the steady rest to the workpiece is released; and the steady rest is moved along the longitudinal axis of the workpiece and, if desired, chucked and clamped again at a different position. In this case, too, there is a program-controlled rotation of the steady rest, so that a collision of the open clamping elements or other regions of the steady rest with the workpiece is prevented.
After the clamping elements are chucked and clamped into this position and, as a result, are fixed, the rotary part of the steady rest can be rotated with the workpiece about its longitudinal axis in order to ensure machining on the periphery.
It is also possible that the steady rest is arranged between the clamping points during the rotating machining of the workpiece and chucks the workpiece and rotates synchronously to the drives of the clamping points by means of a rotary drive, so that torsional stresses in the workpiece are reduced and prevented.
The device for machining elongate, non-rotationally symmetrical workpieces in the form of turbine blades, wherein a first clamping point for clamping a first end of the workpiece and a second clamping point for clamping a second end of the workpiece are provided, and the workpiece is supported by means of a steady rest, which can be moved along a longitudinal axis of the workpiece and has clamping elements for clamping the workpiece on its non-rotationally symmetrical cross section, said clamping elements being arranged on a rotary part and can be moved between a closed clamping position, with which the workpiece can be clamped, and an open position, in which the workpiece is released, wherein, according to the invention, the rotary part has a separate rotary drive and that the device has a control unit for the program-controlled rotation of the rotary part of the steady rest when the steady rest is moved along a longitudinal axis of the workpiece, wherein the clamping elements are arranged in the open position during the movement of the steady rest.
The steady rest can be moved in an advantageous way along the longitudinal axis of the first and the second clamping points by means of a slide and the associated guides. In this case the longitudinal axes of the clamping points are coaxial to each other and to the longitudinal axis/the machining axis of the workpiece.
Owing to the fact that the workpiece can be moved over the first and/or the second clamping point, in this position the region between the first and the second clamping point is accessible for inserting and removing the workpiece, as a result of which the aforementioned and described advantages can be achieved.
To this end the steady rest has a breakthrough, which guarantees a through-passage of the workpiece as well as the first and/or the second clamping points.
In order to clamp the workpiece in the region that is located between the end-sided clamping points, the steady rest has one or more clamping elements, which can be moved between a closed clamping position, with which the workpiece can be clamped, and an open position, so that a clearance between the clamping elements guarantees the through-passage of the first and/or the second clamping points through the steady rest.
The steady rest has a rotary part, which can be rotated with the workpiece about its longitudinal axis. The rotary part is mounted in a rotatable manner in a base body and has clamping elements for clamping the workpiece.
The rotary part and the base body are designed in such an advantageous way that both the rotary part and the base body are closed on the periphery, as a result of which a high rigidity of the steady rest is achieved. This circumferentially closed design is only possible, if the steady rest does not have to be moved transversely to the workpiece, but rather moved over one of the clamping points in order to insert the workpiece.
The clamping elements of the steady rest can be connected to or can be separated from the corresponding hydraulic elements in a docking position, so that it is guaranteed that the rotatable part of the steady rest can be rotated unimpeded in the base body after the separation from the hydraulic elements.
The rotary part is provided with a separate rotary drive in an advantageous way and can be moved along the longitudinal axis of the first and the second clamping points by means of a slide and the associated guides. In this case the longitudinal axes of the clamping points are coaxial to each other and to the longitudinal axis/the machining axis of the workpiece.
The invention provides a surprisingly simple possibility for the use of a steady rest. In this case it is possible to achieve an unimpeded insertion of the workpiece and still retain the distance between the clamping points. Furthermore, the steady rest is not moved transversely to the workpiece, but rather along the longitudinal axis, so that a simple and compact insertion for machining elongate, non-rotationally symmetrical workpieces, in particular, turbine blades, is provided.
The invention is explained in detail below by means of one exemplary embodiment and with reference to the associated drawings.